Also:
* Add three explicit sizing modes to Stack for non-positioned
children: loose, expand, and passthrough. (All three are used.)
* Fix a bug whereby layers would try to paint in the same frame as
they were removed from layout (but not detached).
* Fix a bug whereby Offstage wasn't properly marking the parent dirty
when changing its sizedByParent flag.
* Explicitly make Overlay expand non-positioned children.
* Explicitly have InputDecoration pass through the constraints from
its Row to its Stack children.
Use the same technique for updating compositing bits as layout and
painting. This avoids walking the entire rendering tree when all you
need to update is a small subtree.
